A color image is formed in a color photographic element by generation of an imagewise distribution of the dye as a function of silver halide development. It is common practice to remove the silver image formed on development and the silver halide remaining in unexposed areas of the element. This is accomplished by a bleaching step in which the silver image is oxidized by a suitable oxidizing agent, commonly referred to as a bleaching agent, followed by dissolving the silver halide in a silver halide solvent, commonly referred to as a fixing agent. Alternatively, the bleaching agent and the fixing agent can be combined in a bleach-fixing solution and the silver removed in one step by the use of such a solution.
Various compounds known as bleach accelerators are often used to accelerate the bleaching process. One class of compounds, substituted thiols, have been found to accelerate bleaching either when directly added to the bleaching solution or, if introduced via a prebath, before the bleaching step. See, for example, U.S. Pat. No. 3,893,858.
U.S. Pat. No. 4,163,669 suggests that there is an advantage to incorporating a bleach accelerator directly in a photographic film but that incorporation of a free thiol would have adverse effects on the photographic properties of the film. U.S. Pat. No. 4,163,669 suggests incorporating in a photographic element certain bleach accelerators as salts of a heavy metal ion. Among the bleach accelerators mentioned are the organic thiols of the type described in U.S. Pat. No. 3,893,858.
In order to obtain bleach accelerator silver salt dispersions which can be readily incorporated into a silver-halide photographic film and which remains in stable suspension during storage, the particle size of the bleach accelerator silver salt crystals should be relatively small, for example, less than 3 microns. This small particle size has heretofore been obtained by catastrophically precipitating the bleach accelerator silver salt crystals and then incorporating them into photographic elements by a variety of techniques. Especially preferred techniques include homogenizing or ball milling a slurry of the compound in the presence of a surfactant to form finely divided particles, as disclosed in Swank et al U.S. Pat. No. 4,006,025; milling a mixture of molten compound and a molten or liquid dispersing agent, as described in British Patent No. 1,251,590; or mechanically dispersing the compound, as described in Belgian Patent No. 852,138. Ultrasound can be employed to dissolve the compound prior to its incorporation in the photographic coating composition, as illustrated by Owen et al. U.S. Pat. No. 3,485,634 and Salmien U.S. Pat. No. 3,551,157. Alternatively, the compound can be dispersed directly in a hydrophilic colloid such as gelatin; or the compound can be loaded into latex and dispersed, as illustrated by Chen, Research Disclosure, Vol. 159, 1977, Item 15930.
Apparatus and procedures for introducing and blending bleach accelerator compounds are illustrated by Johnson et al U.S. Pat. Nos. 3,425,835; 3,570,818; 3,773,302 and 3,850,643; McCrossen et al U.S. Pat. No. 3,342,605, Collins et al U.S. Pat. No. 2,912,343 and Terwilliger et al U.S. Pat. Nos. 3,827,888 and 3,888,465.
U.S. Pat. No. 4,865,956 discloses a bleach accelerator silver salt having the structure: ##STR1## which, when incorporated in a photographic element, permits the element to be used in a multiplicity of bleach and bleach-fix compositions- The method used to incorporate the compound in the photographic element was catastrophic precipitation followed by ball milling as described in Swank et al., U.S. Pat. No. 4,006,025.
The above processes provide dispersions which may be difficult to reproduce. Further, dispersions prepared by these methods may contain a large amount of less active species of a bleach accelerator silver salt which can cause greatly reduced accelerating activity.
There is a need to produce a bleach accelerator silver salt compound with more consistent particle morphology and size, and hence, more reproducible bleach acceleration and photographic quality. There is further a need to produce a bleach accelerator silver salt dispersion which contains little or no inactive species. It is also desirable to produce it using a fast and efficient method.